Process for producing pulp and at least one organic liquid or liquefiable material of value with offgas recycling

ABSTRACT

The present invention relates to an integrated process for producing pulp and at least one organic liquid or liquefiable material of value, wherein
         a) a lignocellulose-comprising starting material is provided and subjected to digestion with an aqueous-alkaline treatment medium,   b) a cellulose-enriched fraction and a cellulose-depleted black liquor are isolated from the digested material,   c) the black liquor is subjected to a treatment to give at least one organic liquid or liquefiable material of value and at least one offgas stream,   d) at least one of the offgas streams obtained in step c) is recycled to the process for producing pulp and utilized.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing pulp and at least one organic liquid or liquefiable material of value, in which an offgas stream obtained in the production of the material of value is recycled to the pulp process and utilized there.

The large amounts of biomass produced continually by nature have hitherto been used to only a small extent as renewable raw material for use as material or for the generation of energy. To conserve resources of raw materials, processes which allow the replacement of fossil raw materials by biomass starting materials are required.

In order to achieve high efficiency, attempts are made to use as much of the biomass material provided as possible.

With a contribution of about 700 billion metric tonnes of the estimated biomass reserves of 1.5 trillion metric tonnes on earth, cellulose is the most important representative in the group of organic biopolymers and a very versatile raw material. However, in the biomass available as raw materials source, cellulose rarely occurs in pure or sufficiently enriched form, but essentially as a constituent of lignocellulose. The chemical digestion of lignocellulose results in a mass known as pulp which comprises predominantly cellulose. Pulp is the basis for the production of wood-free paper which does not yellow. The pulp for paper is predominantly produced from wood chips, but other plant fibers can also be used.

There are two main types of pulp processes which dominate the market: the acid sulfite process (the Mitscherlich method) and the alkaline sulfate process. Nowadays, it is mainly the sulfate process, also referred to as kraft process, which is used throughout the world. It is named after the Na₂SO₄ which is added as “make-up chemical” in the recovery of the digestion chemicals; the actual active substances are sodium hydroxide and sodium sulfide.

There is a further need for a process for pulp production, in which the lignin present in the lignocellulose-comprising starting material is efficiently passed to a high-value use. As regards the materials circuits of the process chemicals and solvents used, the lignin recovery and further processing should be integrated as well as possible into the process for pulp production.

The preparation of aromatic compounds having a low molecular weight and especially phenolic compounds from lignin-comprising starting materials is known. Such aromatic compounds having a low molecular weight and especially phenolic compounds have found wide use as intermediates and products of value. They serve, for example, as precursor for various resins, surface-active compounds, specialty chemicals, etc. However, there is a further need for a simple, inexpensive process which allows the preparation of a variety of aromatic products for various fields of use. It is advantageous for further materials of value to be able to be obtained in addition to the desired aromatic products and if possible be used again in the process for aromatics production or a process coupled therewith, e.g. a process for pulp production.

Subjecting streams from various digestion processes of lignin- or lignocellulose-comprising materials to an after-treatment to isolate materials of value is known.

WO 2006/031175 describes a process for isolating lignin from a black liquor, in which this liquor is acidified and dewatered in order to precipitate the lignin.

U.S. Pat. No. 2,057,117 describes a process for preparing vanillin, in which a starting material selected from lignocellulose, a crude lignin extract and ligninsulfonic acid is heated with an aqueous alkali metal hydroxide solution under superatmospheric pressure and the reaction mixture obtained is admixed with sulfuric acid in order to precipitate organic constituents and convert the vanillin into a soluble form.

WO 99/10450 describes a process for converting lignin into a hydrocarbon fuel. Here, lignin is subjected to a base-catalyzed depolymerization and subsequently hydroprocessing. This hydroprocessing comprises a hydrodeoxygenation and mild hydrocracking. The latter is carried out under conditions under which partial hydrogenation of the aromatic rings occurs.

WO 2008/027699 A2 describes a process in which lignin originating from a pyrolysis of biomass is, after water-soluble constituents have been separated off, decarboxylated and hydrodeoxygenated and the organic products from this process step are subsequently subjected to hydrocracking.

WO 2010/026244 describes an integrated process for producing pulp and at least one low molecular weight material of value, in which

a) a lignocellulose-comprising starting material is provided and subjected to digestion with a treatment medium,

b) a cellulose-enriched fraction and at least one cellulose-depleted fraction are isolated from the digested material, where the cellulose-depleted fraction comprises at least part of the treatment medium from step a),

c) the cellulose-depleted fraction is subjected to a treatment to give at least one low molecular weight material of value, and

d) the material/materials of value is/are isolated from the treatment product obtained in step c).

In an embodiment of the process, a cellulose-enriched fraction and a lignin-enriched fraction are isolated from the digested material, the lignin-enriched fraction is subjected to a depolymerization and an aromatics composition is isolated from the depolymerization product.

WO 2009/108601 describes a process for producing a starting material for biorefinery processes for producing a biofuel from a lignin-comprising starting material. Here, lignin from a black liquor of the pulping process or else the black liquor itself is subjected to hydroprocessing in the presence of a hydrogen-comprising gas and a catalyst on an amorphous or crystalline oxidic support. Specifically, a heterogeneous molybdenum sulfide catalyst is used. When black liquor is used, the hydroprocessing can also be carried out in two stages. The process can either be carried out at a refinery site to which the lignin or black liquor is transported or directly on the site of a paper mill. The biorefinery process following hydroprocessing is not described in more detail.

WO 2009/108599 has a disclosure content comparable to WO 2009/108601, with the focus on paper production.

In Angew. Chem. 2008, 120, 9340-9351, M. Stöcker describes the catalytic conversion of lignocellulose-rich biomass to produce BTL (biomass-to-liquid) fuels in biorefineries. The use of a lignin material obtained from the biomass in a pyrolysis to produce biooil and a further work-up to give phenolic resins, synthesis gas, etc., is also shown schematically.

US 2009/0227823 describes a process for producing at least one liquid hydrocarbon product from a solid hydrocarbon starting material (e.g. a lignocellulose material), in which the starting material is subjected to catalytic pyrolysis and the pyrolysis products are subjected to a catalyzed after-reaction to give liquid products.

In Chem. Rev. 2006, 106, 4044-4098, G. W. Huber et al. describe synthesis of fuels from biomass. According to that article, lignocellulose materials can in principle be converted into liquid fuels by means of three routes which differ in their primary step: gasification of synthesis gas, pyrolysis to biooil, hydrolysis to give sugars and lignin. The biooils obtained in the pyrolysis can subsequently be subjected to hydrodeoxygenation in the presence of hydrogen or to steam reforming.

It has surprisingly been found that the production of further organic liquid or liquefiable materials of value from the black liquor obtained can advantageously be integrated into the kraft process (sulfate process) for pulp production. In addition, an offgas stream can be obtained in the production of the further materials of value and this can in turn be recycled to the process for producing pulp and be utilized in that process.

BRIEF SUMMARY OF THE INVENTION

The invention provides an integrated process for producing pulp and at least one organic liquid or liquefiable material of value, wherein

a) a lignocellulose-comprising starting material is provided and subjected to digestion with an aqueous-alkaline treatment medium,

b) a cellulose-enriched fraction and a cellulose-depleted black liquor which comprises at least part of the treatment medium from step a) are isolated from the digested material,

c) the black liquor is subjected to a treatment to give at least one organic liquid or liquefiable material of value and at least one offgas stream,

d) at least one of the offgas streams obtained in step c) is recycled to the process for producing pulp and utilized.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a preferred embodiment of the treatment of the black liquor according to the process of the invention; and

FIG. 2 shows vaporization of an aromatics-comprising stream.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, an organic liquid or liquefiable material of value is an organic compound or a composition comprising at least two organic compounds which are liquid or can be liquefied without decomposition under standard conditions (0° C., 1013 mbar). Here, liquefaction refers to the transition from the solid state into the liquid state in the sense of melting and not solubilization with addition of a solvent.

The organic liquid or liquefiable material of value is, for example, selected from unfunctionalized and functionalized aliphatic, cycloaliphatic and aromatic hydrocarbons. These include especially alkanes, (e.g. pentane, hexane, etc.), alkenes, alkadienes, alkanols (e.g. methanol, ethanol, etc.), aliphatic aldehydes (e.g. acetaldehyde, etc.), cycloalkanes, cycloalkenes, cycloalkadienes, cycloalkanols, cycloalkadienols, cycloalkane polyols having more than two OH groups and unfunctionalized and functionalized aromatic hydrocarbons.

The organic liquid or liquefiable material of value is preferably selected from unfunctionalized and functionalized aromatic hydrocarbons. Functionalized aromatic hydrocarbons preferably have at least one substituent selected from C₁-C₄-alkyl, OH, C₁-C₄-alkoxy, formyl, C₁-C₄-acyl and combinations thereof. The organic liquid or liquefiable material of value is in particular selected from benzene, alkylated benzenes (e.g. toluene and xylenes), relatively highly condensed aromatic hydrocarbons, monoalkylated, dialkylated and polyalkylated relatively highly condensed aromatics, phenol, monoalkylated, dialkylated and polyalkylated phenols, relatively highly condensed aromatics having one, two or more than two OH groups, monoalkylated, dialkylated and higher-alkylated, relatively highly condensed aromatics having one, two or more than two OH groups, alkoxylated derivatives of the abovementioned aromatic alcohols and mixtures thereof.

In a specific embodiment, the organic liquid or liquefiable material of value prepared according to the invention is an aromatics composition having a high content of single-ring aromatics which are unalkylated or have a low degree of alkylation. For the purposes of the present invention, an aromatics composition having a high content of single-ring aromatics which are unalkylated or have a low degree of alkylation is a composition which, based on its total weight, comprises at least 50% by weight of single-ring aromatics. The content of unalkylated and monoalkylated aromatics is, based on its total weight, at least 50% by weight.

The organic liquid or liquefiable material of value prepared according to the invention preferably comprises at least 70% by weight, particularly preferably at least 80% by weight, in particular at least 90% by weight, based on the total weight, of single-ring aromatics.

For the purposes of the present invention, “black liquor” is a cellulose-depleted fraction from the aqueous-alkaline digestion of a lignocellulose-comprising starting material by the kraft process (sulfate process) for pulp production.

The “black liquor” used in step c) can also be a lignin-comprising material obtained by removing at least part of the compounds other than lignin from the black liquor isolated in step b) before the further treatment in step c). The fraction obtained from the black liquor by removal of at least part of the compounds other than lignin will hereinafter also be referred to as “lignin-enriched fraction”. Suitable lignin-comprising materials for use in step c) are pure lignin and lignin-comprising compositions from a black liquor. Here, the lignin content is not critical within a wide range, although if the lignin content is too low, the process can no longer be operated economically.

For the purposes of the invention, “pyrolysis” is a thermal treatment of the black liquor or a lignin-enriched fraction obtained from the black liquor, with molecular oxygen not being introduced or introduced only in a small amount. Here, a small amount is an amount which is significantly smaller than the amount necessary for complete oxidation of the carbon comprised in the material used for the pyrolysis to CO₂. The amount of molecular oxygen introduced in the pyrolysis is preferably at least 50 mol % below, particularly preferably at least 75 mol % below, in particular at least 90 mol % below, the amount necessary for complete oxidation of the carbon comprised in the material used for the pyrolysis to CO₂. The pyrolysis generally occurs endothermically.

For the purposes of the invention, “dealkylation” refers to a reaction of the substituted and/or polycyclic aromatic compounds comprised in an aromatics composition in the presence of hydrogen and/or water vapor, with these being at least partly transformed in such a way that substituents are replaced by hydrogen and/or compounds comprising a plurality of aromatic rings are cleaved to give compounds having a lower number of rings. The substituents replaced by hydrogen are selected from alkyl groups, hydroxy groups, alkoxy groups, aryloxy groups, etc. For the purposes of the present invention, the term “dealkylation” also refers to reactions different therefrom which are associated with a decrease in the molecular weight, e.g. dehydroxylation, dealkoxylation, aromatics cleavage. The term aromatics cleavage refers to a reaction in which essentially the number of aromatic rings per molecule is reduced without the aromatic rings themselves being destroyed.

For the purposes of the invention, the term “offgas stream” refers to gas streams of various compositions as are obtained in the treatment of the black liquor for producing the organic liquid or liquefiable material of value.

“Standard conditions” are, for the purposes of the invention, 0° C. and 1013 mbar (in accordance with DIN 1343).

In a first embodiment, the black liquor or a lignin-enriched fraction obtainable from the black liquor is subjected to a pyrolysis to effect decomposition. This generally results in organic components which cannot be vaporized under the pyrolysis conditions. These remain, for example, in the catalyst bed or are present on the inert particulate added material of a fluidized-bed reactor. The combustion of these unvaporizable components results in a pyrolysis gas which is suitable for recycling as offgas stream. In this first embodiment, the offgas stream comprises at least one of the components CO₂, CO, H₂O, O₂, SO₂ and mixtures thereof.

In a second embodiment, the black liquor or a lignin-enriched fraction obtainable from the black liquor is subjected to a pyrolysis to effect decomposition. The pyrolysis products formed are subjected to a separation step, e.g. condensation or absorption. A fraction which is depleted in organic liquid or liquefiable materials of value and is gaseous under the process conditions can result, and this is suitable for recycling as offgas stream. In an alternative embodiment, a liquid or solid fraction as “off-material” stream which is suitable like an offgas stream for recycling to the process for producing pulp can also result. A liquid or solid fraction can, for example, be obtained when the (originally gaseous) pyrolysis products are subjected to an additional separation into at least two fractions in which the off-material is obtained in liquid or solid form. Such an off-material stream then represents an offgas stream for the purposes of the invention. (All streams originally obtained in gaseous form in the process can be offgas streams for the purposes of the invention).

In a third embodiment, the black liquor or a lignin-enriched fraction obtainable from the black liquor is subjected to a treatment to give a treatment product which is subsequently separated off and subjected to a dealkylation. The dealkylation product is subjected to a separation to give at least one organic liquid or liquefiable material of value and at least one stream enriched in components which are more volatile than the organic material of value. The stream enriched in components which are more volatile than the organic material of value is suitable as offgas stream. In this third embodiment, the offgas stream comprises H₂, H₂O, CO, CO₂, volatile organic components such as methane, H₂S, etc.

In a fourth embodiment, the black liquor or a lignin-enriched fraction obtainable from the black liquor is subjected to a treatment to give a treatment product and the treated material obtained is subsequently subjected to a dealkylation. The dealkylation product is subjected to a separation to give a stream E1) enriched in single-ring aromatics which are unalkylated or have a low degree of alkylation and a stream E2) which is enriched in aromatics which are not dealkylated or have a low degree of dealkylation. The stream E2) enriched in aromatics which are not dealkylated or have a low degree of dealkylation is suitable as offgas stream. In this embodiment, the offgas stream comprises one or more components selected from monoalkylated, dialkylated and polyalkylated phenols; alkoxyphenols such as methoxyphenols; polyalkylated benzenes; compounds which comprise two or more aromatic rings and mixtures thereof. These components will hereinafter be referred to as “aromatics which are not dealkylated or have a low degree of dealkylation”. The stream E2) can be subjected to a further separation to give a fraction which is enriched in compounds which can essentially not be dealkylated under the process conditions.

In a fifth embodiment, the offgas stream has the composition of synthesis gas. The term synthesis gas is explained below.

In a specific embodiment, the offgas stream comprises at least one sulfur compound, in particular H₂S.

For the purposes of the invention, the term “synthesis gas” refers to a gas mixture comprising carbon monoxide and hydrogen. This gas mixture can additionally comprise further gases such as CO₂, CH₄, etc. The process of the invention advantageously makes it possible to produce synthesis gas having a high content of carbon monoxide and hydrogen.

Digestion by the Kraft Process (Step a)

According to the invention, a lignocellulose-comprising starting material (lignocellulose material) is provided and subjected to digestion by the kraft process (sulfate process) in step a) of the process. The digestion makes possible at least partial separation of the lignocellulose-comprising starting material into cellulose and materials accompanying cellulose. The materials accompanying cellulose include lignin, hemicelluloses, silicates, extractable low molecular weight organic compounds (known as extractables, e.g. terpenes, resins, fats), polymers such as proteins, nucleic acids and vegetable gum (known as exudate), etc. These materials accompanying cellulose are generally constituents of the black liquor isolated in step b).

Lignin is a biopolymer whose basic unit is essentially phenylpropane which, depending on the natural source, may be substituted by one or more methoxy groups on the phenyl rings and by a hydroxy group on the propyl units. Typical structural units of lignin are therefore p-hydroxyphenylpropane, guaiacylpropane and syringylpropane which are joined to one another by ether bonds and carbon-carbon bonds.

The lignocellulose materials which can be used in step a) can be obtained, for example, from wood fibers and plant fibers as starting material. Preferred lignocellulose materials are those from wood and residues of the wood processing industry. They include, for example, the various types of wood, e.g. broadleaved tree wood such as maple, birch, pear tree, oak, alder, ash, eucalyptus, common beech, cherry tree, lime, nut tree, poplar, willow, etc. and conifer timbers such as Douglas fir, spruce, yew, hemlock, pine, larch, fir, cedar, etc. Wood can be divided not only into wood from broadleaved trees and that from conifers but also into “hardwoods” and “softwoods”, which is not synonymous with the terms broadleaved tree timbers and conifer timbers. In contrast to hardwood, the term softwood refers to lighter wood (i.e. wood having a dried density below 0.55 g/cm³, for example willows, poplars, limes and virtually all conifers). All hardwoods and all softwoods are in principle suitable for use in the process of the invention. The wood used in the process of the invention can also be present in manufactured form, e.g. in the form of pellets. Suitable residues from the wood-processing industry are not only scrap wood but also sawdust, parquetry grinding dust, etc. Further suitable lignocellulose materials are natural fibers such as flax, hemp, sisal, jute, straw, coconut fibers, switchgrass (Panicum virgatum) and other natural fibers. Suitable lignocellulose materials are also obtained as residues in agriculture, e.g. in the harvesting of cereal (wheat straw, maize straw, etc.), maize, sugarcane (bagasse), etc. Suitable lignocellulose materials are also obtained as residue in forestry, e.g. in the form of branches, bark, wood chips, etc. Another good source of lignocellulose materials is short rotation crops which allow high biomass production on a relatively small area.

According to the invention, a lignocellulose-comprising material is subjected in step a) to digestion in an aqueous-alkaline treatment medium by the kraft process (sulfate process).

It can be advantageous to subject the lignocellulose-comprising starting material to at least one pretreatment step before the digestion. Such steps include, for example, mechanical comminution of the cellulose-comprising starting material, e.g. by shredding and/or milling. Due to their materials properties, fibrous materials are preferably not subjected to pressure-shear comminution but to impact comminution. Suitable milling apparatuses are hammer mills, milling apparatuses operating according to the principle of jet milling and beater mills. The latter are especially suitable for high throughputs.

It can be advantageous to treat the lignocellulose-comprising starting material with mineral acid and/or steam before digestion using the aqueous-alkaline treatment medium. Suitable mineral acids are, for example, hydrochloric acid and in particular sulfuric acid. Treatment with steam is preferably carried out at a temperature in the range from about 110 to 300° C., particularly preferably from 120 to 250° C. Treatment of the lignocellulose-comprising starting material with mineral acid and/or steam before digestion with an aqueous-alkaline treatment medium brings about at least partial hydrolysis of the hemicelluloses comprised in the lignocellulose material. In the case of conifer timbers, generally from 10 to 15% by weight of the lignocellulose material, based on the total weight, goes into solution in the prehydrolysis. In the case of wood from broadleaved trees, generally from 15 to 20% by weight of the lignocellulose material, based on the total weight, goes into solution in the prehydrolysis.

The digestion in step a) is carried out by the kraft process (sulfate process). The treatment medium used in step a) then comprises, as main components, at least one base and at least one alkali metal sulfide in an aqueous medium.

Suitable bases are alkali metal and alkaline earth metal hydroxides, e.g. sodium hydroxide, potassium hydroxide, calcium hydroxide or magnesium hydroxide, alkali metal and alkaline earth metal hydrogencarbonates, e.g. sodium hydrogencarbonate, potassium hydrogencarbonate, calcium hydrogencarbonate or magnesium hydrogencarbonate, alkali metal and alkaline earth metal carbonates, e.g. sodium carbonate, potassium carbonate, calcium carbonate or magnesium carbonate, alkaline earth metal oxides such as calcium oxide or magnesium oxide, and mixtures thereof. Preferred bases are sodium hydroxide, potassium hydroxide and mixtures thereof. Sodium hydroxide is particularly preferred as base.

The treatment medium used in step a) preferably comprises NaOH and at least one sulfur-comprising compound, preferably Na₂S and/or NaHS, in an aqueous medium.

In a specific embodiment, the treatment medium used in step a) comprises NaOH, Na₂S and/or NaHS, Na₂CO₃ and Na₂SO₄ in an aqueous medium.

The digestion of the lignocellulose-comprising starting material using at least one aqueous-alkaline treatment medium in step a) is preferably carried out at a temperature in the range from 50 to 300° C., particularly preferably from 70 to 250° C.

In a specific embodiment, the temperature is increased stepwise or continuously during the course of the digestion in step a) until the desired final temperature has been reached. For this purpose, the digestion can, for example, be carried out at a temperature in the range from about 50 to 130° C. in a first stage and in a range from about 130 to 250° C. in a second stage. The duration of the first stage is, for example, from 5 to 50% of the total digestion time. Heating is carried out using conventional apparatuses, e.g. by means of heat exchangers, heating baths, gas burners, etc. The use of heat obtained in other parts of the process of the invention, e.g. from the combustion of the offgas stream obtained in the production of the organic liquid or liquefiable material of value, is also possible.

The pressure in the digestion in step a) is generally in the range from 0.1 bar to 100 bar, preferably from 1 bar to 10 bar. In a specific embodiment, the digestion is carried out at ambient pressure.

The duration of the digestion in step a) is generally from 0.5 minutes to 7 days, preferably from 5 minutes to 96 hours.

The digestion in step a) can be carried out in one or more stages. In the simplest case, the digestion in step a) is carried out in one stage. In a suitable embodiment of a two-stage digestion, a subsequent stage can, for example, have a higher temperature and/or a higher pressure than the preceding stage. In a multistage digestion, the digested material from only one of the stages or from a plurality of stages can be used for the further processing in step b). However, a condition for this is that the material used for further processing in step b) is a black liquor.

The treatment medium used in step a) is capable of solubilizing at least part of the materials accompanying cellulose in the lignocellulose-comprising starting material under the digestion conditions indicated in more detail below. Here, in particular, an at least partial, preferably essentially complete, solubilization of the lignin comprised in the lignocellulose-comprising starting material takes place. This means that preferably at least 50% by weight, particularly preferably at least 75% by weight, based on the total weight of the lignin comprised in the lignocellulose-comprising starting material, is solubilized. The cellulose comprised in the lignocellulose-comprising starting material is not solubilized or solubilized to only a small extent in the treatment medium. This means that preferably not more than 20% by weight, particularly preferably not more than 10% by weight, based on the total weight of the cellulose comprised in the lignocellulose-comprising starting material, is solubilized.

For the purposes of the invention, the term “solubilization” refers to conversion into a liquid state and comprises the generation of solutions of materials accompanying cellulose (especially lignin-comprising solutions) and also conversion into a different solubilized state. If a lignocellulose constituent is converted into a solubilized state, the individual molecules, e.g. polymer molecules, do not necessarily have to be completely surrounded by a solvate shell. The important thing is that the lignocellulose constituent goes over into a liquid state as a result of the solubilization. Solubilizates for the purposes of the invention thus also include colloidal solutions, microdispersions, gels, etc.

Isolation of Cellulose and Black Liquor (Step b)

In step b), a cellulose-enriched fraction and a cellulose-depleted black liquor are isolated from the digested material.

In step b), the isolation of the cellulose-enriched fraction and the cellulose-depleted fraction(s) is preferably effected by filtration, centrifugation, extraction, precipitation, distillation, stripping or a combination thereof. A person skilled in the art will be able to control the composition of the cellulose-depleted black liquor via the isolation method. The isolation of the cellulose-enriched fraction and the black liquor in step b) is preferably carried out by filtration or centrifugation.

Customary filtration processes are, for example, cake filtration and deep bed filtration (e.g. as described in A. Rushton, A. S. Ward, R. G. Holdich: Solid-Liquid Filtration and Separation Technology, VCH Verlagsgesellschaft, Weinheim 1996, pages 177ff., K. J. Ives, in A. Rushton (editor): Mathematical Models and Design Methods in Solid-Liquid Separation, NATO ASI Series E No. 88, Martinus Nijhoff, Dordrecht 1985, pages 90ff.) and crossflow filtrations (e.g. as described in J. Altmann, S. Ripperger, J. Membrane Sci. 124 (1997), pages 119-128). Customary centrifugation processes are described, for example, in G. Hultsch, H. Wilkesmann, “Filtering Centrifuges,” in D. B. Purchas, Solid-Liquid Separation, Upland Press, Croydon 1977, pages 493-559, and in H. Trawinski, Die äquivalente Klärfläche von Zentrifugen, Chem. Ztg. 83 (1959), pages 606-612. Extraction can be carried out using, for example, a solvent which is not miscible with the treatment medium from pulp production or at least one solvent which has a miscibility gap and in which lignin and optionally further desired components are soluble in a sufficient amount. The removal of components which can be vaporized without decomposition from the lignin-comprising fraction can be carried out by customary distillation processes known to those skilled in the art. Suitable apparatuses for the work-up by distillation comprise distillation columns such as tray columns equipped with bubble caps, sieve plates, sieve trays, ordered packing, random packing elements, valves, side offtakes, etc., evaporators such as thin film evaporators, falling film evaporators, forced circulation evaporators, Sambay evaporators, etc., and combinations thereof.

In a specific embodiment, the removal of at least part of the liquid components from the black liquor is carried out during the process for producing pulp, i.e. in step b). Thus, for example, a black liquor taken off before or during the course of the individual evaporation steps of the parent pulp process can be used for the treatment in step c).

The black liquor obtained in step b) comprises lignin as one of its essential components. The black liquor isolated in step b) further comprises at least one part of the treatment medium from step a).

In general, the black liquor isolated in step b) comprises the following components:

-   -   lignin;     -   hemicellulose;     -   optionally cellulose;     -   optionally organic components other than lignin, hemicellulose         and cellulose;     -   optionally inorganic constituents from the         lignocellulose-comprising starting material;     -   at least one inorganic digestion chemical.

The organic components other than lignin, hemicellulose and cellulose are, for example, selected from degradation products of lignin, hemicellulose and/or cellulose, extractable low molecular weight organic compounds (known as extractables, e.g. terpenes, resins, fats), polymers such as proteins, nucleic acids and plant gum (known as exudate).

The inorganic constituents from the lignocellulose-comprising starting material include silicates.

The inorganic digestion chemicals are selected from those mentioned above under step a). In particular, the inorganic digestion chemicals are selected from NaOH, Na₂S, Na₂CO₃ and Na₂SO₄.

The black liquor has, for example, a solids content in the range from about 10 to 70% by weight.

The black liquor isolated in step b) preferably has a lignin content of from 1 to 50% by weight, particularly preferably from 3 to 30% by weight, based on the solids content of the black liquor.

The black liquor isolated in step b) preferably has a sulfur content of from 0 to 20% by weight, particularly preferably from 0.4 to 4% by weight, based on elemental sulfur and the solids content of the black liquor.

Production of a Material of Value and an Offgas Stream from the Black Liquor (Step c)

In many cases, it is not critical for the isolation of at least one organic liquid or liquefiable material of value from the lignin-comprising black liquor isolated in step b) if the material used for the further treatment comprises at least one further component in addition to lignin.

In a specific embodiment, at least part of the compounds other than lignin is removed from the black liquor before the further treatment in step c). (The fraction obtained from the black liquor by removal of at least part of the compounds other than lignin will hereinafter also be referred to as “lignin-enriched fraction”). The components removed from the black liquor are preferably passed to a further work-up and/or thermal utilization, preferably within the process for cellulose production from which the black liquor was obtained. The components which have been at least partly removed from the black liquor are preferably selected from organic components and/or inorganic process chemicals.

To remove at least part of the compounds other than lignin, the pH of the black liquor can firstly be set to a desired value. For this purpose, the black liquor can be admixed with an acid or an acid-forming component to adjust the pH. Suitable acids and acid-forming components are, for example, CO₂, mineral acids such as hydrochloric acid, sulfuric acid and phosphoric acid. Particular preference is given to CO₂ (or the carbonic acid resulting therefrom by reaction with water). Preference is given to using CO₂ from an offgas stream from the production of the liquid or liquefiable organic material of value or from the integrated pulp process. For example, the offgas from a black liquor combustion (recovery boiler) or a lime kiln is suitable. Here, the offgas can either be introduced directly or after removal of other components (e.g. by means of a scrubbing process such as a Benfield scrub) into the lignin-comprising fraction. Since CO₂ is necessarily obtained in the pulp process, the use of CO₂ for adjusting the pH of the black liquor is associated with lower costs than the use of other acids and also generally allows good integration into the pulp process.

The pH of the black liquor is preferably reduced to a value of not more than 10.5. The isolation of the precipitated lignin is preferably effected by a filtration process. Suitable filtration processes are those mentioned above. If desired, the lignin isolated can be subjected to at least one further work-up step. Such steps include, for example, a further purification, preferably washing with a suitable washing medium. Suitable washing media are, for example, mineral acids such as sulfuric acid, preferably in aqueous solution. In a specific embodiment, a lignin-comprising material can then be provided by firstly acidifying a black liquor from the kraft digestion with CO₂ to precipitate at least part of the lignin comprised, subsequently isolating the precipitated lignin by filtration and subjecting the filtrate to a scrub with sulfuric acid.

A process for isolating lignin from a black liquor by precipitation using CO₂ is described in WO 2008/079072, which is hereby incorporated by reference. The Lignoboost process described in WO 2006/038863 (EP 1797236 A1) and WO 2006/031175 (EP 1794363 A1), which are likewise incorporated by reference, is also particularly suitable.

In step c), the black liquor or the lignin-enriched fraction is preferably subjected to a decomposition for the treatment. The decomposed material obtained in the decomposition of the black liquor or of the lignin-enriched fraction is preferably subjected to a dealkylation for further treatment.

A specific embodiment is a process wherein, in step c):

c1) at least part of the compounds other than lignin is optionally removed from the black liquor to give a lignin-enriched fraction,

c2) the black liquor or the lignin-enriched fraction obtained in step c1) is subjected to a decomposition,

c3) the decomposed material obtained in step c2) is optionally separated into at least one aromatics-enriched fraction C1) and at least one aromatics-depleted fraction C2),

c4) the decomposition product from step c2) or the aromatics-enriched fraction C1) from step c3) is fed into a dealkylation zone and reacted in the presence of hydrogen and/or water vapor,

c5) a discharge is taken off from the dealkylation zone and subjected to a separation, with at least one organic liquid or liquefiable material of value and at least one stream enriched in components which are more volatile than the organic material of value being obtained.

Decomposition

In the decomposition in step c) of the process of the invention, a decomposition product comprising components whose average molecular weight is significantly below the average molecular weight of the components comprised in the lignocellulose-comprising starting material is obtained.

The decomposition product obtained in the decomposition in step c) (especially in step c2) preferably comprises predominantly components having a molecular weight of not more than 500 g/mol, particularly preferably not more than 400 g/mol, in particular not more than 300 g/mol.

The decomposition in step c) (especially in step c2) can in principle be carried out according to two variants, which are described in detail below. The first variant comprises a pyrolysis and correspondingly leads to a pyrolysis product. The second variant comprises a reaction in the presence of a liquid decomposition medium and accordingly leads to a product of the liquid decomposition.

1st Variant: Pyrolysis

In a first variant of the process of the invention, the black liquor or the lignin-enriched fraction is subjected to a pyrolysis in step c) (especially in step c2). In this variant of the process of the invention, the decomposition product is at least partly obtained in gaseous form.

The pyrolysis can be carried out batchwise or continuously. Continuous pyrolysis is preferred.

The pyrolysis is carried out in at least one pyrolysis zone. The black liquor or the lignin-enriched fraction can be introduced into a pyrolysis zone by means of suitable transport devices, e.g. screw conveyors or pneumatic transport.

To carry out the pyrolysis, the black liquor, especially the lignin-enriched fraction, is preferably used in predominantly solid form. For the purposes of the invention, “predominantly solid form” means that the material can be transported by methods known to those skilled in the art for solids transport (see, for example, Perry's Chemical Engineers' Handbook, edited by Robert H. Perry, Don W. Green, 8th edition, illustrated, McGraw-Hill, 2008). For the pyrolysis, the black liquor, especially the lignin-enriched fraction, is then used as, for example, a moist or predried solid.

The pyrolysis zone can have various embodiments, e.g. as rotary tube furnace or fluidized bed. Both stationary and circulating fluidized beds are suitable. Furthermore, in the fluidized bed, a fluidizing gas (preferably steam, carbon dioxide, nitrogen or a gas mixture from one of the subsequent process steps) and as fluidized material a particulate material which is inert under the prevailing conditions are introduced. A particularly suitable inert material is silica sand. Such a fluidized-bed process is described, for example, in U.S. Pat. No. 4,409,416 A. In an alternative embodiment, the pyrolysis zone comprises at least one fixed bed. The fixed beds can comprise at least one inert fixed bed and/or at least one catalytically active fixed bed. If the process of the invention is operated using at least one fixed bed as pyrolysis zone, operation in cycles, where a pyrolysis phase is followed by a burning-off phase in order to remove relatively nonvolatile components from the fixed bed, can be advantageous.

In a first preferred embodiment, the pyrolysis is not carried out with addition of hydrogen. In this embodiment, the hydrogenating reaction occurs essentially in the dealkylation step (especially in step c4).

In a second preferred embodiment, the pyrolysis is carried out with addition of hydrogen. This embodiment of the pyrolysis can also be referred to as hydrocracking. In hydrocracking, the lignin in the black liquor or the lignin-enriched fraction is completely or partially cleaved into low molecular weight fragments by action of hydrogen. The pyrolysis with addition of hydrogen is preferably carried out in suspension. Furthermore, it is preferably carried out using a catalyst and/or under high pressure. Such a process is described, for example, in U.S. Pat. No. 4,420,644 and in H. L. Chum et al., Adv. Solar Energy, Vol. 4 (1988), 91 ff.

In a further preferred embodiment, an evaporated black liquor from the kraft process is used for the pyrolysis. Such a process is described, for example, in U.S. Pat. No. 3,375,283. The black liquor is in this case present predominantly in solid form. In this process variant, too, a pyrolysis gas stream is obtained as product. The solid residue which is likewise obtained can, for example, be recycled to the pulping process.

The pyrolysis in step c) (especially in step c2) can, if desired, be carried out in the presence of at least one pyrolysis catalyst. Suitable pyrolysis catalysts are, for example, silica, alumina, aluminosilicates, aluminosilicates having sheet structures and zeolites such as mordenite, faujasite, zeolite X, zeolite Y and ZSM-5, zirconium oxide or titanium dioxide.

The temperature in the pyrolysis is preferably in the range from 200 to 1500° C., particularly preferably from 250 to 1000° C., in particular from 300 to 800° C.

The pressure in the pyrolysis is preferably in the range from 0.5 to 250 bar (absolute), preferably from 1.0 to 40 bar (absolute).

The residence time at the pyrolysis temperature can be from a few seconds to a number of days. In a specific embodiment, the residence time at the pyrolysis temperature is from 0.5 seconds to 5 minutes, especially from 2 seconds to 3 minutes. The residence time, especially in a fluidized-bed reactor, is given by the total volume of the reactor divided by the volume flow of the fluidizing gas under the pyrolysis conditions.

Suitable processes for the catalyzed or uncatalyzed pyrolysis of lignin are also described, for example, in WO 96/09350 (Midwest Research Institute, 1996) or U.S. Pat. No. 4,409,416 (Hydrocarbon Research Institute, 1983), which are hereby incorporated by reference.

In the pyrolysis zone, the black liquor or the lignin-enriched fraction is reacted to give a pyrolysis product which under the conditions of the pyrolysis is present at least partly in gaseous form (“pyrolysis gas”). Furthermore, a pyrolysis product which under the conditions of the pyrolysis is present partly in liquid and/or solid form can result from the pyrolysis.

The composition of the pyrolysis product can vary as a function of the black liquor or the lignin-enriched fraction.

In all cases, the decomposition product obtained in the pyrolysis in step c) (especially in step c2) comprises substituted aromatics and/or polycyclic aromatics. The decomposition product can comprise, in addition to aromatics, further components selected from water vapor, inert gas (e.g. nitrogen), nonaromatic hydrocarbons, H₂, CO, CO₂, sulfur-comprising compounds such as H₂S, SO₂, etc., and mixtures thereof. The nonaromatic hydrocarbons are preferably degradation products such as methane.

The separation and further treatment of the decomposition product obtained in the pyrolysis in step b) will be described in more detail below (especially for step c3).

According to the invention, an offgas stream is obtained in step c) of the process and is subsequently recycled (in step d) to the process for producing pulp and utilized therein. If step c) comprises a pyrolysis, this generally results in organic components which cannot be vaporized under the pyrolysis conditions. These remain, for example, in the catalyst bed or are present on the inert particulate material of a fluidized-bed reactor. Combustion of these nonvaporizable components results in a burning-off gas which is suitable for recycling as offgas stream. In this embodiment, the offgas stream comprises at least one of the components CO₂, CO, H₂O, O₂, SO₂ and mixtures thereof.

To obtain the offgas stream, a discharge comprising nonvolatile or slightly volatile components from the pyrolysis zone can, after separating off pyrolysis gas in a suitable apparatus (e.g. cyclone), be brought into contact with an oxygen-comprising gas at elevated temperatures.

As an alternative, the offgas stream can be obtained by interrupting the pyrolysis of the black liquor or the lignin-enriched fraction and bringing the pyrolysis zone itself into contact with an oxygen-comprising gas at elevated temperatures in order to remove nonvolatile or slightly volatile components.

Preference is given to using air as oxygen-comprising gas. The temperature for obtaining the offgas stream is preferably in the range from 400 to 2000° C., particularly preferably from 600 to 1500° C., in particular from 700 to 1300° C.

The burning-off of the nonvolatile or relatively nonvolatile components formed in the pyrolysis gives the offgas stream. The burning-off can be carried out in the pyrolysis zone itself or in a separate burning-off zone.

If the process of the invention is operated using at least one fixed bed as pyrolysis zone, the burning-off is preferably carried out in a separate burning-off zone. The inert material is then separated off from the resulting off-material stream by means of a suitable separation apparatus or in the pyrolysis zone itself (configuration as, for example, fluidized bed) and recycled via a suitable transport device to the pyrolysis zone. However, the burning-off can also be carried out at intervals, in which case a pyrolysis interval is in each case followed by a burning-off interval. In the burning-off interval, relatively nonvolatile components are removed from the fixed bed and the offgas stream is obtained in this way.

2nd Variant: Decomposition in the Liquid Phase

In a second variant of the process of the invention, the black liquor or the lignin-enriched fraction is subjected to decomposition in the presence of a liquid decomposition medium in step c) (especially in step c2). In this variant, the decomposition product is obtained at least partly in the liquid state.

The decomposition in the liquid state can in principle be carried out by many methods which differ primarily in terms of the decomposition medium. The black liquor or the lignin-enriched fraction is preferably subjected to decomposition in the presence of an aqueous-alkaline decomposition medium. In particular, the decomposition medium is the digestion medium from step a) of the process of the invention. The appropriate information given under step a) is hereby fully incorporated by reference at the present point.

The decomposition in the liquid state in step c) (especially in step c2) is preferably carried out above ambient temperature. The temperature is preferably in the range from about 40 to 300° C., particularly preferably from 50 to 250° C. In a specific embodiment, the temperature is firstly increased stepwise or continuously during the course of the treatment until the desired final temperature has been reached.

The decomposition in the liquid state in step c) (especially in step c2) can be carried out under reduced pressure, at ambient pressure or above ambient pressure. The pressure in step a) is generally in the range from 0.01 bar to 300 bar, preferably from 0.1 bar to 100 bar.

The duration of the decomposition in step c) (especially in step c2) is generally from 0.5 minutes to 7 days, preferably from 5 minutes to 96 hours.

The decomposition is preferably carried out in close proximity to the site of pulp production in order to keep the outlay for transport of the cellulose-depleted fraction, especially a black liquor, low. Transport is preferably effected via a pipe.

In all cases, the decomposition product obtained from the decomposition in the presence of a liquid decomposition medium in step c) (especially in step c2) comprises substituted aromatics and/or polycyclic aromatics.

The separation and further treatment of the decomposition product obtained in the presence of a liquid decomposition medium in step c) (especially in step c2) will be described in more detail below (especially in the case of step c3).

In the case of decomposition by pyrolysis, it is possible to use the decomposition product obtained from the decomposition of the black liquor or a lignin-enriched fraction without further separation and/or treatment in a dealkylation. In particular, the decomposition product obtained in step c2) can be used without further separation and/or treatment for the dealkylation in step c4).

In another embodiment of the process of the invention, the decomposition product obtained from the decomposition of the black liquor or a lignin-enriched fraction is subjected to a separation and/or treatment before being used in a dealkylation. In particular, the decomposition product obtained in step c2) is subjected to a separation and/or treatment in step c3) before being used in the dealkylation in step c4).

Separation/Treatment of the Decomposition Product (Step c3)

The decomposed material obtained in step c2) is preferably separated into at least one aromatics-enriched fraction C1) and at least one aromatics-depleted fraction C2) in step c3).

The separation is preferably effected by distillation, extraction, absorption, membrane processes or a combination thereof. The separation is particularly preferably effected by distillation, extraction or a combination thereof.

If the decomposition in step c2) is carried out in the liquid state, the separation in step c) is preferably carried out by means of distillation and/or extraction.

In a first specific embodiment of the process of the invention, the black liquor or a lignin-enriched fraction thereof is subjected in step c2) to a decomposition in the liquid state and the separation into at least one aromatics-enriched fraction C1) and at least one aromatics-depleted fraction C2) in step c3) comprises an extraction and/or a distillation.

Before the separation in step c3), the pH of the discharge from a decomposition in the liquid state in step c2) is preferably set to a desired value. According to the invention, a decomposition product which has been obtained by decomposition in the presence of an alkaline digestion medium from the kraft process is used in step c3). The pH is then preferably set to a value of not more than 9, particularly preferably not more than 8, by addition of acid before the separation of the decomposition product. Suitable acids are, for example, mineral acids such as hydrochloric acid, sulfuric acid and phosphoric acid and also acid-forming compounds such as CO₂ and H₂S. Preference is given to using CO₂ from an offgas stream from the process of the invention or the pulp process coupled with the process of the invention. A suitable offgas is, for example, the offgas from a black liquor combustion (recovery boiler) or a lime kiln. Here, the offgas can be introduced into the decomposition product either directly or after removal of other components (e.g. by means of a scrubbing process such as a Benfield scrub). The use of CO₂ for adjusting the pH is thus associated with lower costs than when other acids are used and also generally allows good integration into a pulp process. The hydroxyaromatics obtained in the decomposition in step c2) are virtually entirely present as salts (phenoxides) at pH values above about 9. Effective isolation by distillation and/or extraction is aided by lowering the pH to a value of <9, preferably <8, beforehand.

The separation by distillation of the product obtained from the decomposition in the liquid state in step c2) can be carried out by conventional methods known to those skilled in the art, giving a distillate enriched in aromatics. In this procedure, the steam volatility (heteroazeotropy) of the aromatic fragments obtained in the decomposition in step c2) is utilized to separate these off from the decomposition product. Preference is given to a multistage process in which the heat of condensation of the vapors from at least one stage is utilized for vaporization in another stage. The distillation product obtained is enriched in aromatics compared to the decomposition product used and is suitable, optionally after removal of the aqueous phase, as starting material for the dealkylation in step c4).

The separation of the product obtained from the decomposition in the liquid state in step c2) is preferably also effected by extraction. Here, at least part of the aromatics obtained in the decomposition in step c2) is separated off, while the remaining residue (organic components low in aromatics, inorganic process chemicals, etc.) can be passed to a further work-up and/or thermal utilization, preferably within the process of the invention or an integrated pulp process coupled therewith.

For the extraction, it is possible to use a solvent (extractant) in which the aromatics obtained in the decomposition are soluble in a sufficient amount and which otherwise forms a miscibility gap with the decomposition product. The extractant can then be brought into intimate contact with the decomposition product obtained in step c2), followed by a phase separation. The extraction can have one or more stages.

Suitable extractants are organic compounds such as aromatic or nonaromatic hydrocarbons, alcohols, aldehydes, ketones, amides, amines and mixtures thereof. These include, for example, benzene, toluene, ethylbenzene, xylenes, pentane, hexane, heptane, octane, ligroin, petroleum ether, cyclohexane, decalin, n-butanol, sec-butanol, tert-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, methyl ethyl ketone and mixtures thereof. The extractant preferably has a boiling point at atmospheric pressure which is at least 10 K below the boiling point at atmospheric pressure of that aromatic component which has the lowest boiling point among the components formed to a significant extent in step c2).

The extraction can be carried out continuously or batchwise; see description in: K. Sattler, Thermische Trennverfahren, Wiley-VCH, third revised and expanded edition, July 2001. A plurality of batch separation operations can be carried out one after the other in the manner of a cascade, with the residue separated off from the extractant phase in each case being brought into contact with a fresh portion of extractant and/or the extractant being conveyed in countercurrent. For batch operation, the decomposition product and the extractant are brought into contact with mechanical agitation, e.g. by means of stirring, in a suitable vessel, the mixture is allowed to stand for the phases to separate and one of the phases is removed, advantageously by taking off the heavier phase at the bottom of the vessel. To carry out the extraction continuously, the extractant and the decomposition product are continuously conveyed through suitable apparatuses in a manner analogous to the batchwise variant.

The extraction is carried out, for example, in at least one mixer-settler combination or at least one extraction column. Suitable mixers include both dynamic mixers and static mixers.

In a preferred embodiment, the separation into at least one aromatics-enriched fraction C1) and at least one aromatics-depleted fraction C2) in step c3) comprises the following substeps:

c31) extraction of the decomposition product obtained in step c2) to give an aromatics-enriched extract and an aromatics-depleted residue,

c32) optionally separation of the extract into an extractant-enriched (and aromatics-depleted) fraction and an aromatics-enriched (and extractant-depleted) fraction,

c33) introduction of the aromatics-enriched extract obtained in step c31) or the aromatics-enriched fraction obtained in step c32) into step c4).

Before the extraction, the pH of the decomposition product obtained in step c2) can be adjusted by addition of at least one acid or at least one base. The pH is then preferably set to a value of not more than 9, particularly preferably not more than 8, by addition of acid before separation of the decomposition product. Furthermore, in a multistage extraction, both the pH of the decomposition product used in the first stage and the pH of the residue separated off from the extractant phase (raffinate) in the respective stage can be adjusted by addition of at least one acid or acid-forming compound or at least one base. Suitable acids are, for example, mineral acids such as hydrochloric acid, sulfuric acid and phosphoric acid or acid-forming compounds such as CO₂ and H₂S. Preference is given to using CO₂ from an off-material stream from the process of the invention or the pulp process coupled with the process of the invention. A suitable stream is, for example, the off-material from a black liquor combustion (recovery boiler) or a lime kiln. Here, the off-material can be introduced into the decomposition product either directly or after removal of the other components (e.g. by means of a scrubbing process such as a Benfield scrub). Suitable bases are, for example, alkali metal bases such as sodium hydroxide or potassium hydroxide, alkali metal carbonates such as sodium carbonate or potassium carbonate, alkali metal hydrogencarbonates such as sodium hydrogencarbonate or potassium hydrogencarbonate and alkaline earth metal bases such as calcium hydroxide, calcium oxide, magnesium hydroxide or magnesium carbonate, and also ammonia or amines.

The separation of the extract in step c32) into an extractant-enriched fraction and an aromatics-enriched fraction C1) is preferably carried out by distillation.

The separation by distillation of the extract in step c32) can be carried out by conventional methods known to those skilled in the art. Suitable processes are described in: K. Sattler, Thermische Trennverfahren, Wiley-VCH, third revised and expanded edition, July 2001. Suitable apparatuses for the separation by distillation comprise distillation columns such as tray columns which may be provided with bubble caps, sieve plates, sieve trays, packings, internals, valves, side offtakes, etc. Dividing wall columns, which may be provided with side offtakes, recycling loops, etc., are especially suitable. A combination of two or more than two distillation columns can be used for the distillation. Also suitable are evaporators such as thin film evaporators, falling film evaporators, Sambay evaporators, etc. and combinations thereof.

In a second specific embodiment of the process of the invention, the black liquor or a lignin-enriched fraction derived therefrom is subjected to a pyrolysis in step c2) and the separation in step c3) into at least one aromatics-enriched fraction C1) and at least one aromatics-depleted fraction C2) comprises an absorption.

The discharge taken off from the pyrolysis zone can comprise not only the pyrolysis gases but also proportions of solid and/or liquid components. These can be, for example, relatively nonvolatile components formed in the pyrolysis (e.g. carbonaceous material). If at least one solid inert material is used for the pyrolysis in step c2), the discharge from the pyrolysis zone can also comprise proportions of the inert material. These solid and/or liquid components can, if desired, be separated off from the pyrolysis gas by means of a suitable apparatus, e.g. a cyclone, in step c3). Solid inert materials which have been separated off are preferably recycled to the pyrolysis zone. Components other than inert materials which have been separated off are passed to another use. Such uses include, as described above, for example, combustion to obtain an offgas stream which is according to the invention recycled to the pulp process. It is also possible to bring a discharge from the pyrolysis zone which comprises at least one inert material and components which are relatively nonvolatile under the pyrolysis conditions into contact with an oxygen-comprising gas, preferably air, in a burning-off zone which is physically separate from the pyrolysis zone, leading to burning-off of relatively nonvolatile components (“carbonaceous deposits”) formed in the pyrolysis. The inert material is then separated off from the combustion gas formed by means of a suitable separation apparatus and recycled by means of a suitable transport device to the pyrolysis zone. The combustion gas is at least partly recycled as offgas stream to the pulp process.

If desired, condensable pyrolysis products (i.e. products which are present as liquids or solids under normal conditions) are separated off from the discharge from the pyrolysis (optionally after solids and/or liquids have been separated off). This can be effected by means of suitable separation processes known to those skilled in the art, e.g. condensation, absorption, adsorption, membrane separation processes, etc.

A preferred variant is an absorption in which the discharge from the pyrolysis is, preferably after a cooling step in which a condensation of high-boiling components can also take place, brought into contact with a stream D1) comprising a suitable solvent in a suitable apparatus (e.g. column). A liquid stream D2) comprising the absorption medium and aromatic pyrolysis products and a gaseous stream D3) which is depleted in aromatic pyrolysis products compared to the discharge from the pyrolysis flow out of the contact apparatus. Stream D2) is separated, preferably by distillation, into a fraction D4) which is enriched in aromatic pyrolysis products compared to D2) and a fraction D5) which is depleted in aromatic pyrolysis products compared to D2). D4) is, if necessary after further work-up, fed as stream C1) into the subsequent dealkylation step and D5) is, after further cooling, recycled to the absorption, i.e. D5) is the main constituent of D1). A further constituent is a portion of solvent which is added to make up solvent losses.

Solvents suitable as absorption medium are organic compounds such as aromatic or nonaromatic hydrocarbons, alcohols, aldehydes, ketones, amides, amines and mixtures thereof. They include, for example, benzene, toluene, ethylbenzene, xylenes; pentane, hexane, heptane, octane, ligroin, petroleum ether, cyclohexane, decalin, methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, acetaldehyde, acetone, methyl ethyl ketone, N-methylpyrrolidone, dimethylformamide, dimethylacetamide and mixtures thereof.

The solvent preferably has a boiling point which is below that of the phenol under identical conditions. The solvent particularly preferably has a boiling point which is at least 10 K below that of the phenol under identical conditions. The solvent preferably additionally has a high solubility in water. Such solvents include, for example, methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol and tert-butanol.

Many of the solvents used as absorption medium have a vapor pressure under the conditions of the absorption which leads to loading of the gas stream D3) leaving the absorption with the absorption medium. This applies especially to the preferably used solvents having a boiling point below the boiling point of phenol. The gas stream D3) exiting from the absorption is then preferably subjected to an at least partial removal of the solvent comprised. The separation of the solvent from the gas stream D3) is preferably carried out in the form of a water scrub. Here, good solubility in water of the solvent used for the absorption is particularly advantageous. The scrubbing water stream loaded with solvent and optionally aromatics can, for example, be worked up by distillation. The absorption medium separated off here is (optionally together with the aromatics) returned to the absorption step.

The decomposition product obtained in step c2) can be subjected in step c3) not only to the above-described separation but also to at least one further treatment step. Additional treatment steps can be carried out before, during or after the separation.

The decomposition product obtained in step c2) or the fraction C1) isolated therefrom in step c3) preferably comprises predominantly components having a molecular weight of not more than 500 g/mol, particularly preferably not more than 400 g/mol, in particular not more than 300 g/mol.

In a specific embodiment of the process of the invention, at least part of the aromatics-depleted fraction C2) isolated in step c3) is used for obtaining the offgas stream.

In a specific embodiment of the process of the invention, at least part of the aromatics-depleted fraction C2) isolated in step c3) is used for producing synthesis gas.

Dealkylation (Step c4)

In the dealkylation, the aromatic degradation products formed in the pyrolysis in step c2) and optionally isolated as fraction C1) in step c3) are at least partly transformed by action of hydrogen and/or water vapor so that substituents are replaced by hydrogen and/or compounds comprising a plurality of aromatic rings are cleaved to form compounds having a smaller number of rings. As indicated above, the term “dealkylation” also refers to reactions in which no alkyl substituent is replaced by hydrogen, e.g. dehydroxylation, dealkoxylation, aromatics cleavage, etc. The substituents replaced by hydrogen are preferably selected from alkyl groups, hydroxy groups and alkoxy groups.

Dealkylation processes suitable for use in step c4) comprise hydrodealkylation, steam dealkylation or mixed forms derived therefrom. In the case of a pure hydrodealkylation in the context of the invention, molecular hydrogen but no water is fed into the dealkylation zone in addition to the pyrolysis gas stream. In the case of a pure steam dealkylation in the context of the invention, water but no molecular hydrogen is fed into the dealkylation zone in addition to the pyrolysis gas stream. The dealkylation process in step c4) can also be configured as a mixed form of hydrodealkylation and steam dealkylation. Both water and molecular hydrogen are then fed into the dealkylation zone in addition to the pyrolysis gas stream. Suitable and preferred process parameters, partly for hydrodealkylation and partly for steam dealkylation, are indicated below. Using this information, a person skilled in the art will be able to determine suitable and preferred process parameters for a mixed form of hydrodealkylation and steam dealkylation. The reaction gas comprising H₂ and H₂O used for the dealkylation then preferably has a mixing ratio of H₂ to H₂O in the range from about 0.1:99.9 to 99.9:0.1. An especially suitable mixing ratio of H₂ to H₂O is in the range from about 40:60 to 60:40.

The hydrogen required for the reaction is, in the case of the steam dealkylation, formed in-situ by reaction of water with (mainly organic) components which are either comprised in the starting mixture for the steam dealkylation or are formed during the steam dealkylation. An example which may be mentioned here is the formation of hydrogen from methane and water according to the equation CH₄+H₂O→CO+3H₂.

The temperature in the dealkylation zone is preferably in the range from 400 to 900° C., particularly preferably from 500 to 800° C.

The absolute pressure in the dealkylation zone is preferably in the range from 1 to 100 bar, particularly preferably from 1 to 20 bar, in particular from 1 to 10 bar.

In a first preferred embodiment, the pyrolysis gas stream is subjected to a hydrodealkylation in step c4). For this purpose, the reaction in step c4) is carried out in the presence of hydrogen.

The temperature in the dealkylation zone for the hydrodealkylation is preferably in the range from 500 to 900° C., particularly preferably from 600 to 800° C.

The absolute pressure in the dealkylation zone for the hydrodealkylation is preferably in the range from 1 to 100 bar, particularly preferably from 1 to 20 bar.

The ratio of the amount of H₂ used to H₂ (stoichiometric) in the hydrodealkylation is preferably in the range from 0.02 to 50, particularly preferably from 0.2 to 10. Here, H₂ (stoichiometric) is the minimum amount of H₂ which is theoretically required for complete conversion of the aromatics fed into the dealkylation zone into benzene, with the assumption that 1 mol of H₂ reacts per ring substituent.

The residence time in the dealkylation zone for the hydrodealkylation is preferably in the range from 0.1 to 500 s, particularly preferably from 0.5 to 200 s.

In a second preferred embodiment, the pyrolysis gas stream is subjected to a steam dealkylation in step c4). For this purpose, the reaction in step c4) is carried out in the presence of water vapor.

The temperature in the dealkylation zone for the steam dealkylation is preferably in the range from 400 to 800° C., particularly preferably from 475 to 600° C., in particular from 525 to 600° C.

The absolute pressure in the dealkylation zone for the steam dealkylation is preferably in the range from 1 to 100 bar, particularly preferably from 1 to 20 bar, in particular from 1 to 10 bar.

The ratio of the amount of H₂O used to C* in the steam dealkylation is preferably in the range from 0.1 to 20 mol/mol, particularly preferably from 0.5 to 2 mol/mol. C* is the molar amount of carbon determined by carbon-based balancing of the pyrolysis or by determination of the amounts of product discharged from the steam dealkylation by means of methods known to those skilled in the art.

The molar ratio of H₂ to CH₄ in the dealkylation zone of the steam dealkylation is preferably <50:1, particularly preferably <25:1.

In a steam dealkylation in the absence of a dealkylation catalyst, the molar ratio of OR (where R=H, alkyl) to C_(total) in the dealkylation zone is preferably >0.05:1, particularly preferably from 0.1:1 to 0.2:1.

In a steam dealkylation in the absence of a dealkylation catalyst, the ratio of OR (where R=H, alkyl) to C_(eliminable) in the dealkylation zone is preferably >0.5:1, particularly preferably from 1:1 to 10:1, in particular from 1:1 to 2:1.

The WHSV in the steam dealkylation is preferably in the range from 0.05 to 10 kg/l*h, particularly preferably from 0.1 to 2 kg/l*h.

The steam dealkylation can be carried out in the presence or absence of a catalyst. In a specific embodiment, the steam dealkylation is carried out in the absence of a catalyst. A catalyzed process for steam dealkylation is described in WO 2008/148807 A1. This document and the references cited therein is, in respect of suitable catalysts, hereby fully incorporated by reference. Further information on catalyst types and process steps for steam dealkylation may be found in WO 2007/051852 A1, WO 2007/051851 A1, WO 2007/051855 A2, WO 2007/051856 A1, WO 2008/135581 A1 and WO 2008/135582 A1 (EP 2008055585), without being restricted thereby. U.S. Pat. No. 3,775,504 states that a steam dealkylation actually comprises a combination of steam dealkylation and hydrodealkylation, since it is inherent in the system that at least part of the hydrogen produced is immediately reacted again.

At least one low molecular weight aromatic material of value is formed as target product of the process of the invention in the dealkylation step c4). The low molecular weight aromatic materials of value are preferably selected from benzene and phenolic compounds such as phenol and/or dihydroxybenzenes.

They have, in particular, smaller proportions of the following components than the pyrolysis discharge before introduction into the dealkylation step c4): monoalkylated, dialkylated and polyalkylated phenols; alkoxyphenols such as methoxyphenols; polyalkylated benzenes; compounds comprising two or more aromatic rings. These components will hereinafter be referred to as “aromatics which are not dealkylated or have a low degree of dealkylation”.

Separation of the Discharge from the Dealkylation Zone (Step c5)

A discharge is taken from the dealkylation zone and subjected to a separation. Here, at least one organic liquid or liquefiable material of value is obtained as first product of value and at least one stream enriched in components which are more volatile than the organic material of value is obtained as second product of value. Preference is given to obtaining an aromatics composition having a high content of single-ring aromatics which are unalkylated or have a low degree of alkylation as first product of value.

The discharge from the dealkylation zone is preferably subjected to a separation to give the following three streams:

E1) a stream enriched in single-ring aromatics which are unalkylated or have a low degree of alkylation,

E2) a stream enriched in aromatics which are not dealkylated or have a low degree of dealkylation,

E3) a stream enriched in by-products which are more volatile than E1) and E2).

The discharge from the dealkylation zone can optionally be subjected to a separation to give further streams, e.g. a water-comprising stream.

Stream E1) is the first product of value produced in the process of the invention. E1) is preferably an aromatics composition having a high content of single-ring aromatics which are unalkylated or have a low degree of alkylation. In addition, stream E1) can be subjected to a further work-up to give the aromatics composition produced according to the invention.

Stream E1) preferably comprises, based on the total amount of E1), at least 70% by weight, particularly preferably at least 80% by weight, in particular at least 90% by weight, of single-ring aromatics.

Stream E1) preferably comprises, based on the total amount of E1), not more than 30% by weight, particularly preferably not more than 20% by weight, in particular not more than 10% by weight, of aromatics which are not dealkylated or have a low degree of dealkylation.

As mentioned above, the term “dealkylation” also refers, for the purposes of the invention, to the replacement of substituents other than alkyl groups (e.g. alkoxy groups, aryloxy groups, hydroxy groups, etc.) by hydrogen. Depending on the composition of the aromatics used for the dealkylation, stream E1) then also has a high content of aromatics in which a substituent different from alkyl groups has been replaced by hydrogen. In particular, stream E1) has a high content of aromatics which are unalkoxylated or have a low degree of alkoxylation.

Stream E2) preferably comprises, based on the total amount of E2), at least 70% by weight, particularly preferably at least 80% by weight, in particular at least 90% by weight, of aromatics which are not dealkylated or have a low degree of dealkylation.

The stream E2) can be subjected to a further separation to give a fraction which is enriched in compounds which essentially cannot be dealkylated under the process conditions. This fraction enriched in dealkylation-resistant compounds is then preferably recycled as offgas stream to the pulp process.

Stream E3) comprises components which are, for example, selected from nonaromatic hydrocarbons, especially methane, hydrogen, carbon monoxide, carbon dioxide and mixtures thereof. In addition, stream E3) generally comprises sulfur-comprising by-products from the kraft process, especially H₂S.

The stream E3) is preferably recycled at least partly as offgas stream to the pulp process. In a specific embodiment, at least part of the stream E3) is used for producing synthesis gas. The synthesis gas obtained in this way or a hydrogen-rich gas stream produced therefrom can in turn be recycled at least partly as offgas stream to the pulp process.

As process for the separation, it is possible to use the generally known thermal separation processes.

The separation of the discharge from the dealkylation zone in step c5) preferably comprises an absorption. In the absorption, the gaseous discharge from the dealkylation zone is brought into contact with a solvent (absorption medium), with part of the components comprised in the gas stream being absorbed and thus separated off.

The absorption is carried out in a suitable apparatus, e.g. a countercurrent column, bubble column, etc. The absorption is preferably carried out in a countercurrent column.

The absorption can have one or more stages.

The absorption is preferably carried out using a solvent (not loaded: absorbent, loaded: absorbate) in which the aromatics obtained in the dealkylation are soluble in a sufficient amount and the relatively volatile by-products different therefrom are essentially insoluble. The aromatics which are not dealkylated or have a low degree of dealkylation are at least partly absorbed together with the single-ring aromatics which are unalkylated or have a low degree of alkylation (=target product).

The absorption thus gives an absorbate loaded with aromatics. The aromatic components comprised in the absorbate correspond in terms of their composition to the sum of the aromatics in streams E1) and E2) plus optionally aromatics comprised in the absorption medium. The components comprised in the remaining gas stream correspond in terms of their composition to stream E3). If desired, the gas stream can be subjected to an additional purification step to remove aromatics. These can then once again be combined with the aromatics comprised in the solvent separated off for joint work-up. However, such an isolation of aromatics from the gas stream separated off is generally not necessary.

In a preferred embodiment, the separation of the discharge from the dealkylation zone in step c5) comprises the following substeps:

c51) contacting of the discharge from the dealkylation zone with an absorption medium to give an aromatics-enriched absorbate and a gas stream E3) depleted in aromatics,

c52) separation of the absorbate into a stream E1) enriched in single-ring aromatics which are unalkylated or have a low degree of alkylation, a stream E2) which is enriched in aromatics which are not dealkylated or have a low degree of dealkylation and optionally a stream comprising the absorption medium,

c53) if present, recycling of the stream comprising the absorption medium to step c51),

c54) optionally recycling of at least part of the stream E2) to the dealkylation zone of step c4).

The absorption medium preferably has a boiling point which is above that of the components of stream E1. If the dealkylation is operated so that predominantly benzene is formed, the boiling point at atmospheric pressure of the absorption medium is preferably at least 85° C.; if the dealkylation is operated so that at least some phenol is formed, the boiling point at atmospheric pressure of the absorption medium is preferably at least 187° C.

In a first suitable embodiment, use is made of an absorption medium which is different from the components of streams E1) and E2). The absorption medium preferably has a boiling point which is above that of the components of the stream D1. Furthermore, the absorption medium preferably has a high solvent capability for the aromatics formed in the dealkylation step. Suitable solvents are, for example, aliphatic, cycloaliphatic and aromatic hydrocarbons, aliphatic, cycloaliphatic and aromatic alcohols, amides such as N-methylpyrrolidone or dimethylformamide. Aliphatic, cycloaliphatic and aromatic hydrocarbons preferably have at least 6 carbon atoms. Aliphatic, cycloaliphatic and aromatic alcohols preferably have at least 4 carbon atoms.

When an absorption medium which is different from the components of streams E1) and E2) is used, the separation of the discharge from the dealkylation zone in step c5) preferably comprises the following substeps:

c51) contacting of the discharge from the dealkylation zone with an absorption medium to give an aromatics-enriched absorbate and a gas stream E3) depleted in aromatics (or enriched in by-products which are more volatile than E1 and E2),

c52) separation of the absorbate into a stream E1) enriched in single-ring aromatics which are unalkylated or have a low degree of alkylation, a stream E2) which is enriched in aromatics which are not dealkylated or have a low degree of dealkylation and a stream comprising the absorption medium,

c53) recycling of the stream comprising the absorption medium to step e1),

c54) optionally recycling of at least part of the stream E2) to the dealkylation zone of step c4).

In a particularly preferred variant, an aromatics composition which can be obtained by the process of the invention is used as absorption medium. This is especially a mixture of aromatics which have not been reacted or have been incompletely reacted in the dealkylation. In a particularly preferred variant, the absorption medium used is an aromatics composition whose composition corresponds largely to stream E2 or a mixture of E1 and E2.

When an absorption medium whose composition corresponds largely or totally to stream E2 or a mixture of E1 and E2, the separation of the discharge from the dealkylation zone in step c5) preferably comprises the following substeps:

c51) contacting of the discharge from the dealkylation zone with an absorption medium to give an aromatics-enriched absorbate and an aromatics-depleted gas stream E3),

c52) separation of the absorbate into a stream E1) enriched in single-ring aromatics which are unalkylated or have a low degree of alkylation and a stream E2) which is enriched in aromatics which are not dealkylated or have a low degree of dealkylation,

c54) optionally recycling of at least part of the stream E2) to the dealkylation zone of step c4).

In this variant, the solvent can be obtained by partial condensation of the stream from the dealkylation or a gas stream from a preliminary removal of high boilers downstream of the dealkylation. Here, it can be advantageous to insert a further partial condensation in which, in particular, water is condensed out between the abovementioned partial condensation and the absorption. In this variant too, at least partial absorption of the unreacted or incompletely reacted aromatics takes place together with the absorption of the product of value. This means that in this variant, too, the aromatic components comprised in the absorbate correspond in their composition to the sum of the aromatics of streams E1) and E2).

In step c52), the aromatics-enriched absorbate is preferably separated by distillation. The solvent recovered here is, optionally after removal of absorbed water, recycled to the absorption (step c51). The aromatics are processed further as described above and below.

In step c52), the aromatics-enriched absorbate is separated by distillation in at least one column (“regeneration column”). The distillation conditions are preferably selected so that essentially aromatics which are unalkylated or have a low degree of alkylation and, if present, water are obtained as overhead product and essentially the aromatics which are not dealkylated or have a low degree of dealkylation are obtained as bottom product.

It goes without saying that the temperature at the bottom in the separation by distillation in step c52) is selected at such a low value that undesirable secondary reactions of the bottom product are essentially avoided. This can, in particular, be achieved by setting a suitable column pressure and/or the low boiler content in the bottoms (the low boiler content can be reduced further by downstream stripping).

The overhead product obtained in the distillation in step c52) comprises the target product of the process of the invention. It can either be taken off directly as stream E1) or be subjected to a further work-up. Water comprised in the overhead product can be separated off by known methods. The overhead product after condensation of the vapors from the distillation can for this purpose be fed to a phase separator to separate off the water. The resulting water is discharged as a further stream from the process. The organic phase from the phase separator can either be taken off at least partly as stream E1) or be subjected to a further work-up. The organic phase from the phase separator can partly be recycled as runback to the column and/or be subjected to a further work-up by distillation. This preferably serves to remove water still present and/or undesirable organic components.

The bottom product obtained in the distillation in step c52) comprises the aromatics which have not been reacted or not been sufficiently reacted in the dealkylation, i.e. it is enriched in aromatics which are not dealkylated or have a low degree of dealkylation. It can either be taken off directly as stream E2) or be subjected to a further work-up. The bottom product obtained in the distillation in step c52) is preferably divided into at least two substreams. The first substream is preferably recycled as absorption medium to the absorptive separation of the discharge from the dealkylation zone. For this purpose, this substream is, if necessary, cooled to a suitable temperature. A second substream is taken off as stream E2). The stream E2 can be subjected to a removal of constituents which do not belong to stream D2 before recycling to the dealkylation zone of step c4). This is advantageous when, for example, an absorption solvent which is not obtained as intermediate in the process of the invention is used. In addition, it is advantageous also to take off a purge stream from stream E2) at this point and pass this purge stream to, for example, an incineration apparatus in order to reduce the accumulation of components which do not react or react slowly under the conditions of the dealkylation.

The stream E2) is preferably subjected to vaporization before being fed into the dealkylation. A preferred variant is shown in FIG. 2 and explained in the associated description of the figures.

Synthesis Gas Production

In the process of the invention, synthesis gas can be produced as further product of value. In a specific embodiment of the process of the invention, at least part of the aromatics-depleted fraction C2) isolated in step c) (especially in step c3) is used for producing synthesis gas. It is also possible to use an offgas stream from the decomposition (especially from step c2) and/or the dealkylation (especially from step c4) in the production of synthesis gas. This offgas stream can be, for example, a burning-off gas from the combustion of relatively nonvolatile components. The introduction of such an offgas stream enables the H₂/CO ratio of the synthesis gas to be reduced.

The production of synthesis gas preferably comprises the following stages:

-   -   a reforming stage,     -   a converting stage (into which additional water is introduced if         necessary) in which the water gas shift reaction (CO+H₂O         H₂+CO₂) occurs,     -   optionally a stage for the partial removal of acidic gases such         as CO₂.

The way in which the production of synthesis gas is carried out corresponds to the prior art as described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, article “Gas Production”, DOI: 10.1002/14356007.a12_(—)169.pub2.

In a preferred variant, all or part of the synthesis gas produced in the process of the invention is (if necessary after further purification steps known per se to remove water, sulfur-comprising components, CO₂, etc.) used in at least one process which consumes hydrogen, CO or mixtures of the two. Such a process can be, for example, a hydrogenation, hydroformylation, carbonylation, methanol synthesis, synthesis of hydrocarbons by the Fischer-Tropsch process, etc.

In a preferred embodiment of the process of the invention, a synthesis gas-comprising stream produced in the process or a hydrogen-enriched stream produced from the synthesis gas is fed into the decomposition (especially in step c2) and/or into the dealkylation (especially in step c4). Enrichment of the synthesis gas in hydrogen can, as described above, be effected by means of the water gas shift reaction.

Preference is given to feeding a synthesis gas-comprising stream produced in the process or a hydrogen-enriched stream produced from the synthesis gas into the dealkylation in step c4). The particular advantage of this variant is that the proportion of phenol(s) in the products of the dealkylation is higher than in the pure steam dealkylation, i.e. without introduction of hydrogen. This advantage is obtained overall in the dealkylation in the presence of hydrogen. The higher phenol formation represents an economic advantage since phenol is a more valuable material than oxygen-free aromatics such as benzene. In addition, hydrogen which is not produced in the process of the invention is more expensive and in many cases unavailable or obtainable only with a high degree of difficulty, especially when the dealkylation is to be carried out away from an integrated chemical site.

Recycling and Utilization of the Offgas Stream (Step d)

An aim of the pulp process is as complete as possible a recovery of the process chemicals and as complete as possible a thermal utilization of the organic constituents which cannot be utilized in the production of further organic materials of value. In pulp production, the combustion of the black liquor serves to generate power and process heat which cover a considerable part of the energy requirement in pulp and paper production.

The invention provides an advantageous integrated process in which offgas streams obtained in the production of organic materials of value are fed to an apparatus of the pulp process and utilized therein. This is preferably thermal utilization in the form of a combustion. For this purpose, at least one offgas stream is fed to the waste liquor combustion (recovery boiler). Preference is given to the organic liquid or liquefiable material of value being obtained in the vicinity of the site for pulp production. The isolation of the organic liquid or liquefiable material of value and the pulp process are preferably not more than 10 km apart, particularly preferably not more than 5 km apart, in particular not more than 1 km apart. The outlay for transport of the offgas stream (preferably via a pipe) is therefore kept low. This embodiment has the advantage that no additional apparatuses for the treatment or utilization of the offgases are required, namely for combustion (applies especially to components from the dealkylation which are more volatile than the organic material of value and to the synthesis gas), steam or power generation and flue gas desulfurization. The latter is particularly important because the kraft lignin comprises up to 5 percent of sulfur, based on the solids content, which is obtained, in chemically bound form, in the offgas streams.

A preferred embodiment of the treatment of the black liquor according to the process of the invention is shown in FIG. 1.

A black liquor starting material (1), especially a lignin-enriched fraction from the black liquor (1), is subjected to a decomposition.

An offgas stream (2) obtained here is optionally fed to the pulp process, preferably the recovery boiler.

The decomposition product (3) is optionally subjected to a separation and/or treatment in which an aromatics-enriched stream (6) and an aromatics-depleted stream (4) are obtained. The aromatics-depleted stream (4) is optionally at least partly recycled as offgas stream (5) to the pulp process. Furthermore, at least part of the stream (4) can be fed to reforming/converting for synthesis gas production.

The decomposition product (3) or the aromatics-enriched stream (6) obtained therefrom is fed together with a hydrogenation gas stream (7) to a dealkylation unit. The discharge from the dealkylation zone (8) is subjected to a separation into the following 3 streams:

-   -   product of value (stream 9), a material or mixture which has         been formed in the above-described dealkylation,     -   product which has not been dealkylated or has been incompletely         dealkylated (stream 10); this comprises materials which have not         been dealkylated or have been dealkylated to a lesser extent         than the product of value;     -   stream 11 comprising relatively volatile by-products. These are         selected from methane and other hydrocarbons, H₂O, CO, CO₂ and         sulfur-comprising by-products, especially H₂S.

A water stream (water) is optionally separated off in the separation and discharged.

Stream (9) is, optionally after further work-up, taken off as product stream.

The stream (10) enriched in aromatics which are not dealkylated or have a low degree of dealkylation is returned to the dealkylation via a vaporization. A preferred embodiment of the vaporization is depicted in FIG. 2 and described in more detail below. As an alternative, the stream (10) can at least partly be recycled as offgas to the pulp process. For this purpose, the stream (10) can be subjected to a further separation to give a fraction which is enriched in compounds which can essentially not be dealkylated (not shown).

The stream (11) comprising volatile by-products from the separation is at least partly recycled as offgas stream to the pulp process. The stream (11) is preferably fed to a combustion in the pulp process. This can be, in particular, the waste liquor combustion (recovery boiler). This embodiment has the advantage that no additional apparatuses are required for steam or power generation or flue gas desulfurization. In another variant, the combustion is preceded by a desulfurization, e.g. in the form of a hydrogen sulfide-removing gas scrub, followed by conversion of the H₂S into elemental sulfur (e.g. Claus process).

As an alternative, the stream (11) or a substream (12) branched off therefrom can be fed to reforming/converting to produce synthesis gas. Here, optionally with the introduction of a stream comprising water or oxygen (optional stream 13), the organic components comprised in stream (11) are converted into a synthesis gas (14) comprising CO and H₂.

If the dealkylation is a hydrodealkylation, a hydrogen-comprising stream (15) obtained from synthesis gas production can be fed to the dealkylation.

If hydrogen is used for the decomposition, a hydrogen-comprising stream (16) obtained from synthesis gas production can be fed to the decomposition.

It is also possible for synthesis gas which is not required elsewhere to be recycled as offgas stream to the pulp process.

FIG. 2 shows the vaporization of an aromatics-comprising stream as is obtained, for example, as stream E2) (denoted by (10) in FIG. 1) in the separation by absorption and distillation of the discharge from the dealkylation zone. Stream (10) is preferably subjected to vaporization as shown in FIG. 2) before recycling to the dealkylation.

The aromatics stream (10) is preheated in apparatus A to a temperature at which no appreciable decomposition yet occurs in the liquid phase. This preheated stream (stream 100) is fed together with a gaseous stream 200 whose amount, temperature and composition are selected so that the stream 100 is partially or fully vaporized into an apparatus B. The stream 200 comprises reactants for the dealkylation, i.e. in the case of steam dealkylation water vapor and in the case of hydrodealkylation a hydrogen-comprising gas (stream 5 in FIG. 1). The amounts of streams 100 and 200 are set so that a composition favorable for the type of dealkylation selected is obtained in the stream 300 leaving the apparatus B.

Apparatus B is a liquid-gas contact apparatus as is known from the prior art. Apparatus B can be configured, for example, as a vessel having a jet nozzle or a column, with stream 100 being introduced from the top and liquid and gas being conveyed in cocurrent or countercurrent; a relatively nonvolatile residue (stream 250) can optionally be taken off in the lower part. As an alternative, apparatus B can also be configured as a fluidized bed. Additional energy can be introduced efficiently into the stream 100 via the externally heated fluidized material.

In a preferred variant, stream 300 is divided into streams 400 and 500, with stream 400 being recycled to the dealkylation (D) and stream 500 being recycled via a heat exchanger C to apparatus B. This variant allows the temperatures of the streams 100, 200 and 500 (downstream of the heat exchanger) to be limited to limiting values determined by the availability of the heat sources, the thermal stability of the materials in the streams and the stability of the materials of construction. The pressure drop which naturally occurs along the streams 300, 400 and 500 can be compensated by means of a suitable apparatus for compression. Here, it is possible to use generally known compressors or blowers, but it is also possible to configure the apparatus B completely or partially as a liquid jet blower, with stream 100 being used as driving medium. In this case, it is possible, if the amount of stream 100 is not sufficient for the compressing power required, to circulate liquid via apparatus B. 

1. An integrated process for producing pulp and at least one organic liquid or liquefiable material of value, wherein a) a lignocellulose-comprising starting material is provided and subjected to digestion with an aqueous-alkaline treatment medium, b) a cellulose-enriched fraction and a cellulose-depleted black liquor are isolated from the digested material, c) the black liquor is subjected to a treatment to give at least one organic liquid or liquefiable material of value and at least one offgas stream, d) at least one of the offgas streams obtained in step c) is recycled to the process for producing pulp and utilized.
 2. The process according to claim 1, wherein the treatment medium used in step a) comprises NaOH and at least one sulfur-comprising compound, preferably Na₂S and/or NaHS, in an aqueous medium.
 3. The process according to either of the preceding claims, wherein the isolation of the cellulose-enriched fraction and the cellulose-depleted black liquor in step b) is effected by filtration, centrifugation, extraction, precipitation, distillation, stripping or a combination thereof.
 4. The process according to any of the preceding claims, wherein, in step b), a black liquor comprising at least one component selected from lignin; hemicellulose; optionally cellulose; optionally organic components other than lignin, hemicellulose and cellulose; optionally inorganic constituents from the lignocellulose-comprising starting material; inorganic digestion chemicals; and mixtures thereof is isolated from the digested material.
 5. The process according to any of the preceding claims, wherein at least part of the compounds other than lignin is firstly removed from the black liquor in step c) to give a lignin-enriched fraction.
 6. The process according to any of the preceding claims, wherein the black liquor or a lignin-enriched fraction obtained therefrom is subjected to a decomposition for the treatment in step c) and the decomposed material obtained is optionally subjected to a dealkylation.
 7. The process according to claim 6, wherein the black liquor or the lignin-enriched fraction obtained therefrom is subjected to a pyrolysis to effect decomposition and the unvaporizable components obtained under the pyrolysis conditions are used for obtaining the offgas stream.
 8. The process according to either claim 6 or 7, wherein the black liquor or the lignin-enriched fraction obtained therefrom is subjected to pyrolysis to effect decomposition and the decomposition product obtained is subjected to a separation, resulting in a fraction depleted in organic liquid or liquefiable materials of value, which fraction is used at least partly as offgas stream.
 9. The process according to any of claims 6 to 8, wherein the black liquor or a lignin-enriched fraction obtained therefrom is subjected to a decomposition for the treatment in step c) and the decomposed material obtained is subjected to a dealkylation, a discharge is taken from the dealkylation zone and subjected to a separation to give at least one organic liquid or liquefiable material of value and at least one stream enriched in components which are more volatile than the organic material of value and the latter stream is at least partly used as offgas stream.
 10. The process according to any of the preceding claims, wherein a synthesis gas produced in the process is at least partly used as offgas stream.
 11. The process according to any of the preceding claims, wherein, in step c): c1) at least part of the compounds other than lignin is optionally removed from the black liquor to give a lignin-enriched fraction, c2) the black liquor or the lignin-enriched fraction obtained in step c1) is subjected to a decomposition, c3) the decomposed material obtained in step c2) is optionally separated into at least one aromatics-enriched fraction C1) and at least one aromatics-depleted fraction C2), c4) the decomposition product from step c2) or the aromatics-enriched fraction C1) from step c3) is fed into a dealkylation zone and reacted in the presence of hydrogen and/or water vapor, c5) a discharge is taken off from the dealkylation zone and subjected to a separation, with at least one organic liquid or liquefiable material of value and at least one stream enriched in components which are more volatile than the organic material of value being obtained.
 12. The process according to claim 11, wherein the decomposition of the black liquor or a lignin-enriched fraction obtained therefrom in step c2) comprises a pyrolysis.
 13. The process according to claim 12, wherein, in step c3), the separation into at least one aromatics-enriched fraction C1) and at least one aromatics-depleted fraction C2) comprises an absorption.
 14. The process according to claim 11, wherein, in step c2), the black liquor or a lignin-enriched fraction obtained therefrom is subjected to a decomposition in the liquid phase.
 15. The process according to claim 14, wherein, in step c3), the separation into at least one aromatics-enriched fraction C1) and at least one aromatics-depleted fraction C2) comprises an extraction and/or a distillation.
 16. The process according to claim 15, wherein the separation into at least one aromatics-enriched fraction C1) and at least one aromatics-depleted fraction C2) in step c3) comprises the following substeps: c31) extraction of the decomposition product obtained in step c2) to give an aromatics-enriched extract and an aromatics-depleted residue, c32) optionally separation of the extract into an extractant-enriched and aromatics-depleted fraction and an aromatics-enriched and extractant-depleted fraction, c33) introduction of the aromatics-enriched extract obtained in step c31) or the aromatics-enriched fraction obtained in step c32) into step c4).
 17. The process according to any of claims 11 to 16, wherein the aromatics-depleted fraction C2) isolated in step c3) is at least partly used for producing synthesis gas.
 18. The process according to any of claims 11 to 17, wherein the reaction in step c4) comprises a hydrodealkylation or a steam dealkylation or a mixed form derived therefrom.
 19. The process according to any of claims 11 to 18, wherein the temperature in the dealkylation zone is in the range from 400 to 900° C., preferably from 500 to 800° C.
 20. The process according to any of claims 11 to 19, wherein the absolute pressure in the dealkylation zone is in the range from 1 to 100 bar, particularly preferably from 1 to 20 bar.
 21. The process according to any of claims 11 to 20, wherein the discharge from the dealkylation zone in step c5) is subjected to a separation to give the following three streams: E1) a stream enriched in single-ring aromatics which are unalkylated or have a low degree of alkylation, E2) a stream enriched in aromatics which are not dealkylated or have a low degree of dealkylation, E3) a stream enriched in by-products which are more volatile than E1) and E2).
 22. The process according to any of the preceding claims, wherein the separation of the discharge from the dealkylation zone in step c5) comprises an absorption.
 23. The process according to claim 22, wherein the separation of the discharge from the dealkylation zone in step c5) comprises the following substeps: c51) contacting of the discharge from the dealkylation zone with an absorption medium to give an aromatics-enriched absorbate and a gas stream E3) depleted in aromatics, c52) separation of the absorbate into a stream E1) enriched in single-ring aromatics which are unalkylated or have a low degree of alkylation, a stream E2) which is enriched in aromatics which are not dealkylated or have a low degree of dealkylation and optionally a stream comprising the absorption medium, c53) recycling if present, of the stream comprising the absorption medium to step c51), c54) optionally recycling of at least part of the stream E2) to the dealkylation zone of step c4).
 24. The process according to any of claims 21 to 23, wherein the stream E3) obtained in step c5) is at least partly used for obtaining the offgas stream.
 25. The process according to any of claims 21 to 24, wherein the stream E3) obtained in step c5) is at least partly used for producing synthesis gas.
 26. The process according to any of the preceding claims, wherein a synthesis gas-comprising stream or a hydrogen-enriched stream produced from the synthesis gas is at least partly used for obtaining the offgas stream. 